1. Field of the Invention
The present invention relates generally to downhole pumping of fluids, and more particularly, but not by way of limitation, to an improved discharge valve suitable for use, for example, in a downhole petroleum pumping apparatus.
2. Brief Description of Related Art
Downhole pumps are often used to extract petroleum fluids, such as oil and/or natural gas, from subterranean formations when the natural pressure of an oil or gas formation is insufficient to lift or push the petroleum to the surface. One type of pump is known as a sucker rod pump. Such a sucker rod pump generally includes at least a pump barrel, a plunger that travels up and down within the barrel, a sucker rod actuating the plunger, a standing valve positioned at or near a lower end of the barrel, and a traveling valve positioned at or near the end of the plunger so as to travel up and down with the plunger.
A pump chamber is generally formed inside the pump barrel between the standing valve and the traveling valve. The standing valve allows fluid to flow into the chamber, but does not allow fluid to flow out of the chamber. The traveling valve allows fluid to flow out of the chamber, but does not allow fluid to flow into the chamber.
Fluid pumped by a sucker rod pump is preferably substantially all liquid. The plunger is mechanically actuated, for example, by a pumpjack, to move up and down in a reciprocating motion. On the upstroke of the pumping cycle, as the plunger moves from the bottom to the top of a stroke, the hydrostatic pressure of the liquid above the traveling valve forces the traveling valve to close. The upward motion of the traveling valve also causes a negative pressure gradient between the pump chamber and the well bore, across the standing valve. The negative pressure gradient causes the standing valve to open, drawing liquid into the pump chamber.
At the end of the upstroke, the pump chamber is filled with liquid from the formation. When the plunger begins a subsequent downstroke, i.e., moves from the top to the bottom of a stroke, the pressure in the pump chamber is increased, creating a positive pressure gradient from the pump chamber to the pump barrel, which may also be considered a negative pressure gradient across the traveling valve. This pressure gradient causes the traveling valve to open and forces the liquid in the pump chamber to flow through the traveling valve and into the pump jacket. Once the liquid is above the traveling valve and pump barrel, the liquid is forced to the surface by the plunger as the traveling valve is closed during subsequent upstrokes.
One significant drawback to such a sucker rod pump is that the plunger is forced to directly lift the liquid to the surface. Thus, the plunger must support what is generally a very tall column of liquid. This liquid column puts tremendous tensile stress on the sucker rod and can lead to wear and ultimate failure of sucker rods, plungers, and other valve components. To this end, a need exists for downhole pumps and pump components to facilitate extraction of fluids, and especially liquids, from subterranean formations to the surface, while minimizing the forces on pump components.